The present invention is a method for selecting silicon metalloid having improved performance in a direct process for making organohalosilanes. The method comprises (A) heating a chemical grade silicon metalloid sample at a temperature ramp speed controlled to effect a stepwise reduction of oxide impurities present in the chemical grade silicon metalloid to a temperature greater than about 2300.degree. C. in the presence of a carbon source thereby effecting the formation of a reduction product consisting of carbon monoxide and carbon dioxide, (B) determining the amount of the reduction product formed at above a temperature of about 1900.degree. C., and (C) selecting a chemical grade silicon metalloid for use in the direct process for making organohalosilanes based upon the amount of the reduction product formed above a temperature of about 1900.degree. C.
Organohalosilanes, particularly dialkyldichlorosilanes, are important intermediates in the silicone industry. The organohalosilanes are typically hydrolyzed and condensed to form polyorganosiloxanes which can then be processed to form, for example, silicone fluids, elastomers, sealants, adhesives, and resins. The predominant commercial process for preparing these organohalosilane intermediates is one commonly referred to as the "direct process," as originally described by Rochow, U.S. Pat. No. 2,380,995 issued Aug. 7, 1945, and Rochow et al., U.S. Pat. No. 2,380,996, issued Aug. 7, 1945.
Because of the high volume of organohalosilanes used in the silicone industry, considerable effort has been devoted to optimizing the conversion of the silicon metalloid to the diorganodihalosilane, particularly to dimethyldichlorosilane. It is known in the silicone industry that different lots of chemical grade silicon metalloid react differently in the direct process. To attempt to control the lot-to-lot variability of the reactivity of chemical grade silicon metalloid in the direct process, manufacturers of organohalosilanes have set strict controls on the acceptable types and levels of contaminants present in the silicon. Clarke, J., Organometallic Chemistry, 376:165-222 (1989) provides a comprehensive review of the direct process for synthesis of methylchlorosilanes and the effects of contaminants on the process.
Daugherty et al., U.S. Pat. No. 5,427,952, teach a method for analyzing chemical grade silicon intended for use in the direct process for the presence of contaminants, including oxides and carbides of calcium, aluminum, and silicon. The process involves the separation of the contaminants by an alloying process and subsequent analysis of the contaminants for chemical composition.
The present inventors have found that when chemical grade silicon metalloid is heated under controlled conditions to a temperature greater than about 2300.degree. C. in the presence of a carbon source that a reduction product consisting of carbon monoxide and carbon dioxide which forms at above a temperature of about 1900.degree. C. is predictive of the chemical grade silicon metalloid's performance in the direct process. The inventors have found that the smaller the amount of reduction product formed above a temperature of about 1900.degree. C. the greater the specificity of the chemical grade silicon metalloid for the production of diorganodihalosilanes.